Introduce customize_save_after action.

Props dovyp, JustinSainton
fixes #24401


git-svn-id: https://develop.svn.wordpress.org/trunk@24344 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/class-wp-customize-manager.php

@@ -512,6 +512,8 @@ final class WP_Customize_Manager {
 			$setting->save();
 		}
 
+		do_action( 'customize_save_after', $this );
+
 		die;
 	}